the two best marketing words in the united states are "free" and "all you can eat." people love that. so they love free checking, and it brought in a lot of customers that didn't have a checking account before. >> bergman: so why not offer them free checking... >> right. >> bergman: ...and pick up the money... >> other ways. >> bergman: the concept of paying overdrafts-- loaning customers money when their accounts are empty-- quickly caught on at banks across america because it was so profitable. and those profits exploded when the banks began to attach debit cards to those free checking accounts, producing billions of dollars in revenue. they don't charge a fee to give you a debit card? >> they're free until you make a mistake, and then you pay dearly. >> it's just easy when you have a debit card to use it, but one month i didn't get the deposit from my tenant, and i didn't realize it. i bought a small pizza for $7 and ended up getting charged a $33 fee. so it ended up being a $40 pizza. >> bergman: the bank covered the $7 dollar pizza purchase even though her accou